Recent work has been showing the possibility of generating frequency combs at terahertz frequencies using terahertz quantum cascade lasers. The main efforts so far were on getting the laser to work in a stable comb operation over an as broad as possible spectral bandwidth. Another issue is the scattered farfield of such combs due to their subwavelength facets of the used metal-metal waveguide. In contrast to single mode lasers the monolithic approaches of distributed feedback lasers or photonic crystals cannot be used. We present here a monolithic broadband extractor compatible with frequency comb operation based on the concept of an end-fire antenna. The antenna can be fabricated using standard fabrication techniques. It has been designed to support a bandwidth of up to 600 GHz at a central frequency of 2.5 THz. The fabricated devices show single lobed farfields with only minor asymmetries, increased output power along an increased dynamical range of frequency comb operation. A side-absorber schematics using a thin film of Nickel has been used to suppress any higher-order lateral modes in the laser. The reported frequency combs with monolithic extractors are ideal candidates for spectroscopic applications at terahertz frequencies using a self-detected dual-comb spectroscopy setup due to the increased dynamical range along with the improved farfield leading to more output power of the frequency combs.
Markus Rösch, Ileana-Cristina Benea-Chelmus, Giacomo Scalari, Christopher B. Bonzon, Martin J. Süess, Mattias Beck, and Jérôme Faist, "Broadband monolithic extractors for terahertz quantum cascade laser based frequency combs (Conference Presentation)," Proc. SPIE 10123, Novel In-Plane Semiconductor Lasers XVI, 1012313 (Presented at SPIE OPTO: February 01, 2017; Published: 20 April 2017); https://doi.org/10.1117/12.2252175.5398643857001.
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